In distributed communication networks, especially packet-oriented communication networks, a precise time synchronization of the components of the communication network is increasingly required. For this purpose, a “Precision Time Protocol”, known in the technical world as PTP, is used for packet-oriented communication networks, with which real time clocks of spatially-distributed components of the communication network, especially of an ethernet, can be synchronized. The known PTP is standardized in IEEE Standard 1588, which for example is described in the Publication 2004 Conference on IEEE 1588, Sep. 28, 2004, “Implementation and Performance of Time Stamping Techniques, Hans Weibel, Dominic Béchaz
According to this standard, a master, shown in FIG. 3 by a vertical line marked M, is defined in one of the components of the communication network, with a synchronization message SYN-N being transmitted at regular time intervals via a communication network KN to its slaves and/or components and/or devices and/or terminals, shown in FIG. 3. by a vertical line marked S. This synchronization message SYN-N contains the current time tM of the clock arranged in the master M. Because reading the clock, processing the protocol, running through the protocol stacks and transmitting the data through an ethernet controller takes an undefined time, the time information in the synchronization message SYN-N is already out of date when leaving the master M. Therefore, the actual transmission time point of the synchronization message is measured as close as possible to the physical interface, ideally directly at the communication network terminal of the hardware, and this is sent as transmission-time information tMT by means of a follow-up message FUP-N to the slave S. In the standard and in the technical world, this transmission-time information tMT is known as the time stamp. The receiving devices or terminals, i.e. the slaves S, represent the reception-time point and form reception-time information tSE. The reception-time information tSE gives the time point at which the synchronization message SYN N SYNC N was accepted or received from the transmission medium, with the reception time point being determined in the physical layer or in the circuitry. From the sent transmission-time information tMT and the determined reception-time information tSE in the slave S, the deviation tA or the offset of its real time clocks is then determined. The clock of the slaves S is corrected according to the determined deviation tA, i.e. synchronized with the clock of the master M.
If there has been no delay in the transmission path, then both clocks will already be synchronized. A second phase of the synchronization process determines the delay time between the slaves S and the master M, i.e. the measurement of the transit time. For this purpose, the slave S sends a “delay request” or request message DREQ-N to the master M in accordance with IEEE Standard 1588 and for this again determines the exact transmission time point tST. The master M generates a piece of reception-time information tME on receipt of the request message DREQ-N and sends the reception-time information tME back to the slave S in a “delay response” or response message DRES-N. From the local transmitted and received pieces of reception-time information tST, tME or from both time stamps, the slave S determines the delay time or transit time tL in the communication network KN between the slave S and master M. The transit time measurement takes place irregularly and at longer time intervals as a measurement for synchronization of the two clocks.
For determining the reception time points of received synchronization-relevant messages or the transmission time points of the synchronization-relevant messages to be sent, a time stamp unit, which determines the time points or time stamps, is provided in the devices. The determination of the deviations tA of the real time clocks and the transit times tL is performed with the aid of the PTP (Precision Time Protocol), with the PTP being realized in the application layers of the devices or terminals. Because the time points or the time stamps are formed in the physically close layer, in order to be able to determine the deviations tA of the clocks and of the transit time tL in the communication network as accurately as possible the time points or time information is buffer stored in memories of the physical layer until they are called up by the PTP in the application layer. This means that in practice several pieces of time information for several messages and several PTP have to be stored in the memories for several applications in the physical layer. Furthermore, not only is it necessary to store the time information in the memories but a piece of information which enables the assignment of the time information to the respective message also has to be stored, so that the respective PTP in the application layer can appropriately call up the time information from the memories.
For the realization of the PTP according to the IEEE 1588 Standard therefore extensive memories and protocols, to be realized in extensive circuits and programs, are necessary to call up the time information from the memories in the circuit layers or in the hardware layer of the device or terminals of a communication network, such as in an intranet or in the Internet.